Improving HONO Simulations and Evaluating its Impacts on Secondary Pollution in the Yangtze River Delta Region, China

Secondary air pollution, especially ozone (O₃) and secondary aerosols, are emerging air quality challenges confronting China. Nitrous acid (HONO), as the predominant source of hydroxyl radicals (OH), are acknowledged to be essential for secondary pollution. However, HONO concentrations are usually underestimated by current air quality models due to the inadequate representations of its sources. In the present study, we revised the Weather Research and Forecasting & Chemistry (WRF-Chem) model by incorporating additional HONO sources, including primary emissions, photo-/dark oxidation of NO_x, heterogeneous uptake of NO₂ on surfaces, and nitrate photolysis. By combining in-situ measurements in the Yangtze River Delta (YRD) region, we found the improved model show much better performance on HONO simulation and is capable of reproducing observed high concentrations. The source-oriented method is employed to quantitatively understand the relative importance of various processes, which showed that heterogeneous NO₂ uptake on the ground surface was the major contributor to HONO formation in urban areas. Comparatively, photo-oxidation of NO_x is a main contributor in rural areas. The introduction of multiple sources of HONO led to an apparent increase in OH and hydroperoxyl (HO₂) radicals. The promoted HO₂ levels further increased diurnal O₃ concentration by 4.5–12.9 ppb, while secondary inorganic and organic concentrations were also increased by 14–32% during a typical secondary pollution event. The improved description of HONO emission and formation in the model substantially narrowed the gaps between simulations and observations, highlighting the great importance in understanding and numerical representations of HONO in secondary pollution study.